1. Field of the Invention
This invention relates to a radiation image recording and read-out apparatus for recording a radiation image of an object on a stimulable phosphor sheet and photoelectrically read-out the image recorded on the stimulable phosphor sheet.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet comprising the stimulable phosphor is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned with stimulating rays which cause it to emit light in proportion to the radiation energy stored. The light emitted from the stimulable phosphor sheet when the sheet is exposed to the stimulating rays is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image having an improved quality, particularly a high diagnostic efficiency and accuracy. The finally obtained visible image may be reproduced in the form of a hard copy or may be displayed on a cathode ray tube (CRT). In this radiation image recording and reproducing system, the stimulable phosphor sheet is used to temporarily store the radiation image in order to reproduce the final visible image therefrom in a final recording medium. For economical reasons, therefore, it is desirable that the stimulable phosphor sheet be used repeatedly.
Further, in a mobile X-ray diagnostic station such as a traveling X-ray diagnostic station in the form of a vehicle like a bus which is provided with a radiation image recording and read-out apparatus for use in the aforesaid radiation image recording and reproducing system and moves from place to place to record radiation images for mass medical examinations, it is disadvantageous to load the mobile X-ray diagnostic station with a large number of stimulable phosphor sheets, and the number of the stimulable phosphor sheets which can be loaded on the mobile X-ray diagnostic station is limited. Therefore, it is desired to load the mobile X-ray diagnostic station with stimulable phosphor sheets which can be used repeatedly, once store the radiation images of the objects in the stimulable phosphor sheets, transfer the electric image signals read out from the stimulable phosphor sheets into recording medium having a large storage capacity, such as a magnetic tape, and circulate and reuse the stimulable phosphor sheets for further image recording and read-out operations, thereby to obtain the radiation image signals of many objects. Further, when image recording is conducted continuously by circulating and reusing the stimulable phosphor sheets, it becomes possible to increase the image recording speed in mass medical examination. This is very advantageous in practical use.
In order to reuse stimulable phosphor sheets as described above, the radiation energy remaining in the stimulable phosphor sheet after it is scanned with stimulating rays to read out the radiation image stored therein should be erased by exposure to light or heat as described, for example, in Japanese Unexamined Patent Publication No. 56(1981)-12599 or U.S. Pat. No. 4,400,619. The stimulable phosphor sheet should then be used again for radiation image recording.
From the aforesaid viewpoint, the applicant proposed in Japanese Patent Application No. 58(1983)-66730 a built-in type radiation image recording and read-out apparatus comprising:
(i) a circulating and conveying means for conveying at least one stimulable phosphor sheet for recording a radiation image thereon along a predetermined circulation path,
(ii) an image recording section positioned on said circulation path for recording a radiation transmission image of an object on said stimulable phosphor sheet by exposing said stimulable phosphor sheet to a radiation passing through said object,
(iii) an image read-out section positioned on said circulation path and provided with a stimulating ray source for emitting stimulating rays for scanning said stimulable phosphor sheet carrying said radiation image stored therein in said image recording section, and a photoelectric read-out means for detecting light emitted from said stimulable phosphor sheet scanned with said stimulating rays to obtain an electric image signal, and
(iv) an erasing section for, prior to the next image recording on said stimulable phosphor sheet for which the image read-out has been conducted in said image read-out section, having said stimulable phosphor sheet release the radiation energy remaining in said stimulable phosphor sheet, whereby said stimulable phosphor sheet is circulated through said image recording section, said image read-out section and said erasing section and reused for radiation image recording.
In the aforesaid radiation image recording and read out apparatus, recording and read-out of radiation images can be conducted continuously and efficiently. The radiation image recording and read-out apparatus of this type will hereinbelow be referred to as a "built-in type radiation image recording and read-out apparatus".
The built-in type radiation image recording and read-out apparatus is advantageous in various points. For example, when recording radiation images of a number of people at a mobile diagnostic station such as a bus equipped with an X-ray photographing apparatus, the same number of stimulable phosphor sheets as the number of the people must be prepared if the stimulable phosphor sheets are not repeatedly used. However, the number of stimulable phosphor sheets that can be carried on such a mobile vehicle is limited. This problem can be overcome by repeatedly using a stimulable phosphor sheet and storing the image signals read in the image read-out section in a recording medium having a large memory capacity. Further, circulation of the stimulable phosphor sheets facilitates continuous photographing, shortening the time required to record radiation images of a given number of people.
However, with the built-in type radiation image recording and read-out apparatus, there is a problem that fine dust particles floating in the air and produced in the apparatus are apt to settle on the surface of the stimulable phosphor sheet. Dust settling on the side of the stimulable phosphor sheet bearing the stimulable phosphor will prevent light from being emitted from the part of the stimulable phosphor sheet covered with the dust upon exposure to the stimulating rays so that the part is reproduced as a white spot. For example, in a radiation image of a cancer focus, a badly turned mineralized part of the cancer appears as a white spot. Other diseased parts also can appear as white spots. Thus, the part of the body corresponding to the part of the stimulable phosphor sheet covered with the dust can be mistakenly diagnosed as diseased parts.
Even if the surface of the stimulable phosphor sheet is cleaned, dust is apt to settle again on the stimulable phosphor sheet during conveyance thereof since there is usually dust on the circulating and conveying means such as e.g. endless belts or nip rollers, and components associated with the circulating and conveying means. Further, it is difficult and troublesome to clean the circulating and conveying means with cleaning cloth, cleaning liquid and the like due to the complicated structure of the circulating and conveying means.